This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. xc2xa7 119 arising from an application entitled, METHOD AND APPARATUS FOR IDLE HANDOFF IN A CELLULAR YSTEM, earlier filed in the Korean Industrial Property Office on Sep. 9, 1998, and there duly assigned Ser. No. 1998-36976.
The current invention relates to a method for executing an idle handoff in a cellular system and an apparatus thereof. More particularly, a method for executing a hard handoff when a mobile telephone in an idle state moves between base stations having other frequency assignments (FA) in a conventional code division multiple access (CDMA) system and an apparatus thereof.
In a cellular system, there are a plurality of cells within a service area, a plurality of base stations for controlling the cells, and a mobile switching center for controlling the base stations. When a mobile telephone moves between cells, it is possible to maintain the continuity of the transmission traffic by using a handoff function.
A handoff function permits a mobile telephone to continue transmission over a channel to a current base station as the mobile telephone moves into another base station""s coverage region.
FIG. 1 depicts a conventional cellular system using the CDMA technology. As shown in FIG. 1, a conventional cellular system comprises a mobile telephone (MT) 40 for providing call service to user, a base transceiver station (BTS) 30, 31 for providing service to the mobile telephone, a base station controller (BSC) 20 for controlling the BTS, and a mobile switching center (MSC) 10 for connecting between the BSC and a public switched telephone network (PSTN) 11. The MSC 10 receives information on the user from a home location resistor (HLR) 12 and visitor location resistor (VLR) 13.
During an idle state, a mobile telephone monitors the strength of the paging channels transmitted from different base stations.
A mobile telephone receives a message by using one of the paging channels when either receiving an incoming call or initializing an outgoing call. The mobile telephone monitors the various paging channels when moving from a coverage area of a current base station to another coverage area of a neighboring base station. This function for monitoring the paging channels is referred to as an idle handoff.
In a CDMA system, each of the channels transmitted from a base station has a same pseudo-random noise (PN) sequence offset value, which is called a pilot signal, so that the base station is identified by the PN signal. Accordingly, a mobile telephone identifies each of the base stations from the different pilot signals.
A mobile telephone monitors the pilot signal having the strongest intensity from all of the neighboring base stations. When the strength of pilot signal monitored from one of the neighboring base stations is stronger, over a predetermined value (for example, 3 dB), than the strength of the pilot signal of the current base station which is connected to the mobile telephone via the paging channel, the mobile telephone executes the idle handoff at the frequency of the monitored pilot signal and recognizes itself as entering into the new base station.
In a CDMA system, the number of frequency assignments (FA) assigned to a base station is increased as the demand for increased capacity grows.
When a mobile telephone moves from a first base station having a specific channel with a channel frequency into a neighboring base station, which does not have call service at the specific channel frequency of the first base station, or into a neighboring base station which does not have an available channel at the specified channel frequency, a hard handoff is executed. The hard handoff process first cuts off the channel connected between the mobile telephone and base station and makes a new channel connection within a short time in an attempt to prevent a user from recognizing that the hard handoff has occurred.
Although not providing call service at the specified channel frequency, the neighboring base station transmits a dummy pilot signal to the mobile telephone on the specified channel frequency in order to make it possible for the mobile telephone to receive pilot signals at its current specified frequency. Therefore, the mobile telephone can recognize that it has entered into a new coverage area of another base station as soon as it detects a dummy pilot signal.
The current base station, to which the mobile telephone is connected, transmits information on the neighboring base stations to the mobile telephone through a paging channel. This information is called an extended neighbor list.
If the mobile telephone detects a pilot signal from a new base station, and the detected new base station is included in an extended neighbor list, the mobile telephone executes the idle handoff to the new base station. If the new base station is not included in the list, the mobile telephone does not execute the idle handoff.
In FIG. 1, a base station 30 provides call service to a first cell 1, which has two frequency assignments (1FA, 2FA), and is connected to a mobile telephone with a paging channel at the 2FA frequency. A neighboring base station 31 provides call service to a second cell 2 at a frequency assignment (1FA), and the neighboring base station 31 uses the frequency assignment at 2FA for a dummy pilot signal.
As the mobile telephone, which monitors the 2FA frequency paging channel, moves from cell 1 to cell 2, the mobile telephone receives the 2FA dummy pilot signal from the base station 31; this reception indicates that the mobile telephone has entered into the transmission area of cell 2. The mobile telephone subsequently monitors the paging channel of the base station 31 with at the 2FA dummy pilot signal at first and thereafter monitors a paging channel of the base station 31 at the 1 FA frequency so that both an inter-cell handoff (from cell 1 to cell 2), and an inter-frequency handoff (from frequency 2FA to 1 FA) can occur.
As described above, the prior art idle handoff method has some complexities owing to executing two step handoff process including (1) an inter-cell handoff and (2) an inter-frequency handoff within the new cell. Moreover, if a call from a base station or initiated by the mobile telephone user occurs before the inter-cell handoff and inter-frequency handoff are finished, it is impossible to set up the call properly.
It is an object of the current invention to provide an idle state handoff method and a handoff apparatus for a mobile station, which performs a handoff between base stations having different frequency assignments in a cellular system. If a mobile telephone, in an idle state, detects the pseudo pilot signal of the neighboring base stations, then the handoff is directly performed with the frequency provided by the neighboring base stations.
It is another object of the current invention to provide an idle state handoff method and a handoff apparatus for a mobile station in a cellular system, which continuously transmits information to the mobile telephones about the channels and the frequencies of the neighboring base stations.
According to a first preferred embodiment of the present invention, a method for an idle state handoff between base stations having different frequency assignments in a cellular system, the method comprising the steps of:
(a) transmitting information by the current base station to a mobile telephone regarding a frequency assignment of each of a plurality of neighboring base stations and a list of the neighboring base stations building up a channel with the mobile telephone;
(b) detecting a pilot signal from one of the plurality of neighboring base station by the mobile telephone;
(c) determining whether the pilot signal detected from the one of the neighboring base stations is included in the received list of the neighboring base stations;
(d) recognizing that a handoff has failed if the neighboring base station from which the pilot signal is detected is not included in the list of the neighboring base stations;
(e) determining whether a detected pilot signal is a dummy pilot signal by referring to the frequency assignment information, if the detected neighboring base station is included in the list of the neighboring base stations;
(f) deciding a frequency which corresponds to the one neighboring base station to perform a handoff based on the frequency assignment information, if the detected pilot signal is a dummy pilot signal; and
(g) detecting a paging channel from the decided handoff frequency.
In the above embodiment, it is preferable that the information regarding the frequency assignment of the neighboring base stations be transmitted by the current base station includes an index of a pilot pseudo-random noise offset (PILOT_PN), a configuration state of the frequency assignment of the neighboring base station (NGHBR_CONFIG), a frequency indicator to indicate whether a frequency of the mobile telephone connected to the current base station is a dummy pilot signal in the neighboring base station (FREQ_INCL), a frequency band of the frequency assignment (NGHBR_BAND), and an assigned frequency of the frequency assignment (NGHBR_FREQ).
It is preferable that the strength of the detected pilot signal from the neighboring base station is stronger than a pilot signal of the current base station by a predetermined value.
It is also preferable that if the detected pilot signal is not a dummy pilot signal, a step of detecting a paging channel in the present frequency of the neighboring base station.
According to a second preferred embodiment of the present invention, a method for an idle state handoff between base stations having different frequency assignments in a cellular system comprising the steps of:
(a) detecting a new pilot signal from a neighboring base station by a mobile telephone;
(b) determining whether the detected pilot signal is a pilot signal from the neighboring base station;
(c) determining whether the detected pilot signal from the neighboring base station is a dummy pilot signal; and
(i) if it has been determined in step (c) that the detected pilot signal is a dummy pilot signal, deciding a handoff frequency corresponding to the detected neighboring base station and executing a handoff in an idle state at the decided handoff frequency; or
(ii) if it has been determined in step (c) that the detected pilot signal is not a dummy pilot signal, executing an idle state handoff to the neighboring base station at a present frequency.
In the above embodiment, it is preferable that the strength of the detected pilot signal from the neighboring base station is stronger than a pilot signal of the current base station by a predetermined value.
It is also preferable that the above embodiment further includes a step of transmitting by the current base station to a mobile telephone information on a frequency assignment of the neighboring base stations and a list of the neighboring base stations building up a channel with the mobile telephone.
It is preferable that the information on the frequency assignment of the neighboring base station transmitted by the current base station includes an index of pilot pseudo-random noise offset (PILOT_PN), a configuration state of the frequency assignment of the neighboring base station (NGHBR_CONFIG), a frequency indicator to indicate whether a frequency of the mobile telephone connected to the current base station is a dummy pilot signal in the neighboring base station (FREQ_INCL), a frequency band of the frequency assignment (NGHBR_BAND), and an assigned frequency of the frequency assignment (NGHBR_FREQ).
According to a third preferred embodiment of the current invention, a method for idle an idle state handoff in a cellular system comprising the steps of:
(a) detecting by a mobile telephone of a plurality of pilot signals being transmitted from a current base station and from neighboring base stations while the mobile telephone is an idle state;
(b) checking whether one of the detected pilot signals is included in a list of extended neighboring base station including information on a frequency assignment of the neighboring base stations; and
(b) (i) terminating the handoff if the detected pilot signal checked in step (b) is not included in the list of extended neighboring base stations; or
(b) (ii) determining whether the detected pilot signal is a dummy pilot signal if the detected pilot signal checked in step (b) is included in the list of extended neighboring base stations; and
(b) (iii) recognizing a handoff failure if the detected pilot signal is not included in the list of extended neighboring base stations as determined in step (ii);
(b) (iv) performing an inter-cell handoff at a frequency of the detected pilot signal if the detected pilot signal is not a dummy pilot signal as determined in step (ii);or
(b) (v) determining a handoff frequency with reference to the extended neighboring list, performing an inter-cell and an inter-frequency handoff at the determined handoff frequency, and monitoring a paging channel if the detected pilot signal is a dummy pilot signal as determined in step (ii).
According to a fourth embodiment of the present invention, an apparatus for an idle state handoff between base stations having different frequency assignments in a cellular system comprises:
a mobile telephone for detecting pilot signals transmitted from a plurality of neighboring base stations;
means for deciding a frequency by the mobile telephone for the idle state handoff from a list of the plurality of neighboring base stations and information regarding a frequency assignment of each one of the neighboring base stations; means for performing an idle state handoff by the mobile telephone at the decided frequency; and
means for sending to a base station by the mobile telephone the list of the neighboring base stations and information regarding the frequency assignment of the neighboring base stations.
In the above embodiment, it is preferable that the information on the frequency assignment of the neighboring base station transmitted by the current base station includes an index of the pilot pseudo-random noise offset (PILOT_PN), a configuration state of the frequency assignment of the neighboring base station (NGHBR_CONFIG), a frequency indicator to indicate whether a frequency of the mobile telephone connected to the current base station is a dummy pilot signal in the neighboring base station (FREQ_INCL), a frequency band of the frequency assignment (NGHBR_BAND), and an assigned frequency of the frequency assignment (NGHBR_FREQ).
It is preferable that the strength of the detected pilot signal from the neighboring base station is stronger than a pilot signal of the current base station by a predetermined value.